For anything more then a minor edit, I would recommend forking and renaming the persona theme. Forking would allow you to make your own changes, while still having the option to merge or cherry-pick changes to the base repo. If you're not familiar with Git, you could also just make needed changes manually.

@yariplus so I've decided to fork. The thing is, I'm not planning to store the theme on an online service such as GitHub. I think that means I can't fork the repo. My guess is I'd have to clone instead. That way I can keep a local copy, SFTP the files and folders to my online server, and then going forward I could cherry-pick (pull) commits made to Persona that I might need. Is that the right thing to do?